Contact bounce bulb burnout

[al hildenbrand]

I just ran into an anecdotal account describing incandescent bulb failure caused by switch contact bounce. When the switch is turned on, the contacts bounce a number of times resulting in a series of voltage spikes created by the collapse of the cold filament inrush current magnetic field. The resultant voltage spikes hasten the failure of the filament.

Granted that there are numerous other components of incandescent bulb burnout, my question is, is contact bounce bulb burnout a real phenomena?

 

[hbiss]

Re: Contact bounce bulb burnout

...resulting in a series of voltage spikes created by the collapse of the cold filament inrush current magnetic field.

No magnetic field involved, at least in the lamps. Depending on the load, the momentary high inrush current is what's causing the spikes. And, yes, it is inrush current that kills incandescent lamps. You will get longer life by leaving them on rather than turning them off and on all the time. Question is why are the switch contacts bouncing?

 

[al hildenbrand]

Re: Contact bounce bulb burnout

The person relating the anecdote attributed the contact bounce to "cheap 89? switches."

In my mind, a lighting contactor would have the same effect. I have heard that the contacts, when pulled in by the coil, can be shown on an oscilloscope to "bounce" for a brief interval of time.

 

[hbiss]

Re: Contact bounce bulb burnout

All contacts will "bounce" to some degree and as you say can be measured with test equipment. If the bounce is severe enough to cause the problem you relate there is something wrong with the switch or contactor.

 

[bennie]

Re: Contact bounce bulb burnout

I agree about the bounce phenomenon. We need pre-insertion resistors for soft starting. I think a lighted switch will reduce the bounce.

 

[hbiss]

Re: Contact bounce bulb burnout

A lighted switch only puts a neon lamp and current limiting resistor across the contacts. Won't do anything for contact bounce. Like I said, if the bounce is severe enough to cause problems there is something wrong.

I've heard of problems with contactors, where the coil is supplied by the same circuit as the load. A high inrush current will cause the supply voltage to sag thereby causing the contactor to drop out. The load is disconnected and the supply voltage returns to normal. The contactor again pulls in and the process repeats itself. This shouldn't be confused with contact bounce.

 

[hurk27]

Re: Contact bounce bulb burnout

you can get MOV's that will limit the spike to what the bulb voltage is rated at. if the contacts are controling a coil then use the MOV across the coil this will limit the transent at the coil if the bulb socket is mounted on the tranformer then use a bulb rated for a few more volts.

 

[al hildenbrand]

Re: Contact bounce bulb burnout

Going back to the idea of an 89? switch. . .The mass of the contacts is low, say, compared to those of a general purpose relay. And, I dare say, the contact pad surface metallurgy will have less "substance".

The lower contact pad mass, and the slow manual speed of the hand manipulated snap lever, in my mind, makes contact bounce much less of a consideration in snap switches, than magnetic relays.

And with respect to the 89? snap switch lower contact pad mass, I suspect that the "wear and tear" deteriorization of the mating surfaces will do more to sustain the period of inrush current, thereby stressing the bulb filament. That is, dirty, oxidized and/or pitted contacts shorten bulb life.